The concentration of red tide at Collier County beaches are mostly diminishing, according to a Tuesday report from the Florida Fish and Wildlife Conservation Commission.

The FWC’s Fish and Wildlife Institute tested water samples from five Collier beaches —Barefoot Beach State Preserve, Vanderbilt Beach, Seagate Beach, near the Naples Pier and South Marco Beach. Since April 9, red tide concentrations at four of those beaches diminished; concentrations at one remained about the same.

Karenia brevis, the organism that causes red tide, showed up in very low concentrations at Barefoot Beach last week and in Tuesday’s report. The Fish and Wildlife Research Institute found 7,000 Karenia brevis cells per liter of water at Barefoot Beach. That level could cause respiratory problems.

South Marco Beach had low concentrations of Karenia brevis last week and very low concentrations Tuesday.

These samples do not test offshore waters, where red tide could be causing fish kills

Background concentrations of red tide have no effect on people or marine life, according to the FWC. Very low levels can cause respiratory problems and close shellfish harvesting areas. Low concentrations cause fish kills.